Tunnel Blast Design

Tunnelling – Selected Topics

Joint Technical Seminar by AGS(HK), HKIE (Geotechnical Working Group on Cavern & Tunnel Engineering) & HKTS

Tim Magub – Ove Arup & Partners Hong Kong Limited

Key Tunnel Blast Design Parameters

• M2 Face Area

• Tunnel Diameter

• Profile ( Shape )

• Rock Type & Quality

• Blasthole Diameter

• Cut Configuration

• Burden & Spacing

• Allowable Maximum Instantaneous Charge ( MIC )

• Available Delay Range of Non-Electric Detonators

• Delay Sector Initiation Sequence

Small Diameter Water Tunnel

1∼ 10 M 2

Single Track Running Tunnel With Services

Courtesy of Brandrill Engineering ( HK ) Limited

∼ 50 M 2

Single Tube - 3 Lane Road Tunnel

Courtesy of Brandrill Engineering ( HK ) Limited

∼ 220 M 2

Rail Cross-Over Tunnel

Courtesy of Brandrill Engineering ( HK ) Limited

∼ 200 M 2

Blast Face Cut TypesAdvance

Face

Plough - Wedge - V Cut

Burn Cut Types

Spiral Burn Cut

1

2

3

4

5

6

7

8

9

Most Common Cut Types

• Wedge Cuts are largely banned in numerous operations as they eject rock violently, the rock is thrown a considerable distance resulting in services being destroyed e.g. ventilation,power, air and communications.

• Frequently preferred cut type is the Burn Cut• Numerous variations exist and this is largely a ‘ personal

preference ‘ of the Blasting Engineer or Shotfirer.

All Purpose Cut Design – The Shielded Burn Cut

250 ms 150 ms 300 ms

IV II V 45 mm

200 ms

100 ms III

I

VI

350 ms

SHIELDED BURN CUT LAYOUTSection View

Cut Blasthole

Helper Blasthole

1,000mm

250 mm

127 mm

ReliefHole

Shielded Burn Cut

I

II

III

IV V

VI

Shielded Burn Cut Key Features• Relief holes drilled 300mm deeper than blastholes• Cut blastholes are positioned to minimize:

• Sympathetic detonation of adjoining blasthole• Dynamic pressure desensitization of explosive in adjoining

blasthole

• Each loaded cut blasthole• Has two relief holes into which it can break, and• Is shielded ( or protected ) by the relief holes when other cut

blastholes detonate

250 ms 150 ms 300 ms

IV II V 45 mm

200 ms

100 ms III

I

VI

350 ms

SHIELDED BURN CUT LAYOUTSection View

Cut Blasthole

Helper Blasthole

1,000mm

250 mm

127 mm

ReliefHole

The Cut Is Critical For A Successful Blast

• Design the Cut correctly• Position the Cut correctly on the face• Drill the Cut correctly• Load the Cut correctly• Stem the Cut correctly, and• The Cut will ‘ pull ‘ and result in a good blast

• If the Cut does not ‘ pull ‘ the blast advance will be poor• A substantial part of the blast will ‘ freeze ‘• Correcting this failure costs time and money

• The Cut can be placed centrally, or right or left of tunnel centre• The Cut should be placed circa ( 1.5 – 1.8 ) m above the

‘ Lifters ‘• A lower Cut position and early firing Lifter blastholes results in

improved ‘ floors ‘ and maximizes the effect of ‘ gravity ‘ on the remaining blast portions

Burn Cut Minimum Area of LD Blastholes Required

0

50

100

150

200

250

300

350

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

Blasthole Depth ( m )

Min

imu

m X

-Se

cti

on

al A

rea

of

LD

Re

lief

Bla

sth

ole

s (

cm

2 )

Minimum X-Sectional Area of Burn Cut Relief Holes

Bit OD

(mm)

Area

cm2

76 45

89 62

102 82

114 102

127 127

250 ms 150 ms 300 ms

IV II V 45 mm

200 ms

100 ms III

I

VI

350 ms

SHIELDED BURN CUT LAYOUTSection View

Cut Blasthole

Helper Blasthole

1,000mm

250 mm

127 mm

ReliefHole

Better Ejection of the Cut

Potential Freezing of the Face

Minimum Rock Ejection Time For Burn Cut Blastholes

Burn Cut BlastholesMinimum Rock Ejection Time

0

20

40

60

80

100

120

3.0 3.5 4.0 4.5 5.0 5.5 6.0

Round Depth ( m )

Min

imu

m I

nte

r-C

ut

Ho

le

De

lay

Tim

e (

ms

)

Safer Zone

Potential to Freeze Blast

Geological Effects On Tunnel Blast Design

• Varying geological conditions can result in:• Relocating the ‘ Cut ‘ into more competent ground• Expansion or reduction of burdens & spacings• The addition or deletion of a ‘ ring ‘ of blastholes in the face• Adding or deleting individual blastholes from the face

• When a blasthole over-breaks in poor ground, the adjacent blasthole in the next outer ring will produce higher Air Overpressure ( AOP ) due to its thin burden when it detonates

• Higher or lower bulk emulsion explosive density• Longer or shorter ‘ pull ‘ lengths

Tunnel Blast Burden Calculator

Burden CalculatorMost Efficient Starting Point

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

1.25

1.30

0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.20 2.40 2.60

Charge Concentration ( Kg / Lm )

Bu

rden

( m

)

e.g. 45 mm Blasthole

Bulk Emulsion @ 1.00 gms / cm3

Larger Rocks - Poorer Fragmentation

Larger Proportion of FinesInneficient Blasting

Approximate Number of Blastholes Per Face

Source: Jimeno CL, Jimeno EL & Carcedo FJA, 1995 " Drilling & Blasting of Rocks "

Approximate Number of Loaded Blastholes Per Tunnel & Drift Face AreaIncluding Cut, Production, Lifter & Perimeter Blastholes

0

20

40

60

80

100

120

140

0 10 20 30 40 50 60 70 80 90 100 110Tunnel Face Area (m2)

Ap

pro

xim

ate

No

. of

Lo

aded

Bla

sth

ole

s P

er R

ou

nd

27 mm Blastholes

50 mm Blastholes

38 mm Blastholes

32 mm Blastholes

The Importance of Accurate Drilling • Modern Jumbo’s are computer controlled and generally place

blastholes close to their design position in the face, however

• The ‘ power ‘ of modern drifters is high and if the Jumbo operator elects to drill at the highest penetration rate, blastholes can deviate within the face

• This is particularly evident at the 2 and 10 o’clock face position when drilling perimeter blastholes

• The boom is against the tunnel wall, excessive drifter power will bend the drill rod, and

• The blastholes will deviate left at ( 2 o’clock ) and right at ( 10 o’clock )• This usually results in under-break at these locations

210

The Importance of Stemming All Loaded Blastholes

• Stemming blastholes retains the explosive within the blasthole so it does not eject when a nearby blasthole detonates

• When a stemmed blasthole detonates, the stemming momentarily retains the energy within the blasthole, only for a few milliseconds but that is sufficient

• Using crushed rock stemming ( circa 10 mm ) in thin plastic sausages will reduce Air Overpressure ( AOP ) by circa 6 dBL

• Crushed rock stemming will ‘ lock ‘ in the blasthole due to its angular shape

• The use of traditional stemming materials are not effective, e.g.• Wet cardboard• Wet paper• Hessian bags

• Moist clay is not especially effective when compared to crushed rock stemming, as it will not ‘ lock ‘ in the blasthole

The Importance Of Having All Detonators Active Before The First Cut Blasthole Detonates

0 ms Delay Sector

17 ms Delay Sector

9 ms Delay Sector

34 ms Delay Sector

26 ms Delay Sector

43 ms Delay Sector

51 ms Delay Sector

First Cut Blasthole Detonates @ + 100 ms

Last Surface Connector Initiates @ + 51ms

A. All detonator delay elements are burning ( alight ) before the first Cut blasthole detonates at 100 ms

B. This prevents misfires from ‘ cut-offs ‘ due to projectile rocks from a detonating Cut ( or other ) blasthole severing a shock tube before the last Surface Connector initiates

C. As the surface area of the face and the tunnel diameter increases, then more and more sectors must be added to ensure all detonator delay elements are active ( burning ) before the first Cut blasthole detonates

D. The maximum number of sectors is 12 if the first Cut blasthole detonates at 100 ms. The 12th

sector surface connector initiates at 94 ms.

The Importance of Correctly Priming Blastholes

• All primers must be placed at the back ( toe ) of blastholes with the primer and detonator pointing towards the free face

• A primer should have a high Velocity of Detonation ( VOD ) and a high Detonation Pressure ( DP )

• VOD of small diameter ( SD ) Bulk Emulsion : [ 4,800 – 5,200 ] m / sec

• VOD of Blow Loaded ANFO in SD blasthole : [ 2,800 – 2,900 ] m / sec

• Detonation characteristics of selected primers and bulk explosives

Primer Type VOD ( m/sec ) DP ( kBar )

Cartridge Emulsion 4,200 – 4,700 62

Mini Cast Booster 7,000 – 7,800 245

Gelatin Dynamite 4,500 – 4,800 60

SD Blow Loaded ANFO 2,700 – 2,900 40

SD Bulk Emulsion 4,800 – 5,200 90

Steady State Velocity of Explosive Column• The distance & time taken for the explosive column to reach Steady

State Velocity ( SSV ) is a function of the type of primer utilized

• Basic rules for priming• The primer should have a higher VOD than the explosive column• The primer should have a higher DP than the explosive column

• This ‘ overdrives ‘ the VOD of the explosive column and the run-down distance and time to steady state velocity is shortest

• The converse is true for primers with a VOD & DP less than the explosive column, this results in ‘ underdrive ‘, and the run-up distance & time to steady state velocity is longer when compared to‘ overdriving ‘

• The high strength primer diameter should be ≥ 50% of the blasthole diameter which gives the fastest run-down time to steady state velocity of the explosive column ( 67% is the ideal )

• Primer diameters smaller than half the blasthole diameter ( e.g. ⅓ D ) result in longer run-up times and distances to steady state velocity, even if they have a higher VOD & DP than the explosive column

Cast Mini Booster

Higher VOD & DP than Bulk Emulsions

Cartridge Emulsions

Similar VOD to Bulk Emulsion but lower DP

In Sequence Blast Initiation Pattern Design

Courtesy of Brandrill Engineering ( HK ) Limited

Out-Of-Sequence Blast Initiation Pattern Design

Courtesy of Brandrill Engineering ( HK ) Limited

Comparative Tunnel Face Initiation Sequences

Sector Delay

Safer – Easier to correct the ‘ Misfire ‘ Higher Risk – Difficult to correct the ‘ Misfire ‘

Actual Tunnel Face Initiation Sequence

Safer Method – Easier To Recover Misfired Sector(s) Riskier Method – More Difficult To Recover Misfired Sector(s)

Bunch Versus Ring Firing of Sector Blastholes

• ‘ Bunch ‘ firing is bunching all shock tubes in a delay sector together and securing them together with two wraps of 5 gms / m detonating cord

• The practical maximum number of shock tubes in a bunch is 20• If there are more than 20 shock tubes in a delay sector, make 2 x bunches

joined by 2 strands of 5 gms / m detonating cords• Delay sectors are linked by the appropriate non-electric surface delay connector• The surface connector is attached to a 0 ms delay bunch block which initiates

the detonating cord

• ‘ Ring ‘ firing is when all shock tubes within a delay sector are connected to a ring ( circle ) of 2 x 5 gms / m strands of detonating cord

• The detonators have ‘ J ‘ Hooks at their end which clip onto the detonating cord• The ‘ Ring ‘ of detonating cord should be flush with the face and should not

touch any other shock tube tail hanging from an adjacent delay sector• Delay sectors are linked by the appropriate non-electric surface delay connector• The surface connector is attached to a 0 ms delay bunch block which initiates

the detonating cord

Bunch And Ring Firing Initiation

“ Bunch “ Sector Firing‘ Ring ‘ Sector Firing

Detonating Cord Wrap

0 ms Bunch Block

17 ms Surface Connector

Detonating Cord Ring

0 ms Bunch Block

17 ms Surface Connector

Bulk Emulsion Explosive Parameters

• Explosive density can be lowered to give longer charge lengths in areas of reduced Maximum Instantaneous Charge ( MIC’s )

• Has lower ‘ heave ‘ energy but high ‘ shock ‘ energy• Generates lower gas volumes

• Contains ( 2.8 – 3.2 ) Mj / Kg of theoretical thermodynamic energy

Blasthole 0.80 0.85 0.90 0.95 1.00 1.05 1.10

Diameter

38 mm 0.91 0.96 1.02 1.08 1.13 1.19 1.25

42 mm 1.11 1.18 1.25 1.32 1.39 1.45 1.52

45 mm 1.27 1.35 1.43 1.51 1.59 1.67 1.75

48 mm 1.45 1.54 1.63 1.72 1.81 1.90 1.99

51 mm 1.63 1.74 1.84 1.94 2.04 2.14 2.25

64 mm 2.57 2.73 2.90 3.06 3.22 3.38 3.54

gms / cm3

Bulk Emulsion Cup Density

Blasthole Loading Rate - Kg / Lm

Underground Bulk Emulsion Loading Rates

Blow Loaded Bulk Anfo Explosive Parameters

• Density between ( 0.90 – 0.95 ) gms / cm3

• Has lower ‘ Shock ‘ energy but high ‘ Heave ‘ energy• Generates higher gas volumes

• Contains 3.68 Mj / Kg of theoretical thermodynamic energy

• Should only be used in Dry – Damp blastholes

• Is preferred explosive option where conditions allow

0.90 0.95

Blasthole

Diameter

38 mm 1.02 1.08

42 mm 1.25 1.32

45 mm 1.43 1.51

48 mm 1.63 1.72

51 mm 1.84 1.94

64 mm 2.90 3.06

Anfo Cup Density

gms / cm3

Loading Rate - Kg / Lm

UG Blow Loaded Anfo Loading Rates

End